In the remote monitoring system, the monitoring central unit used to be kept in constant touch with the monitoring terminal unit. In recent years, that old system has been replaced with a new one which connects the monitoring terminal unit with the monitoring central unit only when necessary to save the maintenance costs such as communication expenses.
The prior art to the present invention is explained hereinafter.
FIG. 8 shows a block diagram of the remote monitoring system in the prior art. The monitoring image or the output of the TV camera 101 of the monitoring terminal unit 100 is inputted into an encoder 102, where the image is encoded and the image data volume is compressed. The monitoring image thus encoded is referred to an image storage 104 for storage through a terminal control 103.
If the sensor 106 detects something abnormal, the terminal control 103 will activate a communication control 105 to get in touch with the monitoring central unit 200. When that connection is established with the monitoring central unit 200, the monitoring image data stored in the image storage 104 will be sent out successively starting with that of before or after the detection of the abnormality.
The monitoring central unit 200 first stores in an image holder 202 the monitoring image data sent from the monitoring terminal unit 100 through a communication control 201. The image data is then decoded in the decoder 203 and the delayed monitoring image is displayed on a monitor 206. This monitoring image is stored in the image holder 202 and can be reproduced at any time. Another feature is that if the current monitoring data is needed instead of the one stored in the image storage 104, real-time monitoring data can be obtained on the monitoring central unit 200.
In the remote monitoring system of the prior art, the monitoring image obtained in the monitoring central unit 200 is either the real-time image data from the moment of the connection or the delayed image data stored in the monitoring terminal unit 100 before or after the detection of an abnormality.
The prior art system provided the following problem, however: On a real-time basis, it is impossible to get image data before or after the detection of an abnormality which is necessary for tracing the cause thereof, while delayed image data is not a real-time one and is not timely especially in case of emergency. Another shortcoming is that since image data being transmitted is fixed or constant in resolution, there is a possibility that useful monitoring images may not be obtained if the communication service is low in quality. Furthermore, in case there is an interruption in the communication because of some trouble with the communication line, the monitoring image data will be lost up to the moment that the connection is restored.
The present invention is proposed in the light of those shortcomings of the prior art.